Surgery with femtosecond ophthalmic lasers is based on generating a pulsed laser beam and delivering the laser pulses by a scanning delivery system through a focusing optics to a sequence of focus spots along a scan-pattern in a target region of an ophthalmic tissue. Each laser pulse can create a plasma or cavitation bubble in the target tissue at the focus spot of the laser beam when the beam intensity or energy density exceeds a plasma or photodisruption threshold. During surgery, the focus spot of the laser beam is scanned along a three dimensional scan-pattern, creating a sequence of these bubbles to form macroscopic surgical cuts or photodisrupted regions.
During the surgery, however, the laser beam can also cause unintended collateral damage away from the focus spot such as excessive heating and shockwaves in the target tissue and light poisoning in the retina. Therefore, surgical systems are designed to deliver the laser beam with an energy density that exceeds the photodisruption threshold, but only marginally to achieve the surgical functionality while minimizing the collateral damage.
The energy density or beam intensity is determined by the energy, duration and repetition rate of the individual laser pulses and the size of the focus spot. Modern surgical laser systems provide high precision and control by using precisely controlled laser sources, refined optical designs, high quality optical parts and an objective with a large numerical aperture to focus the laser beam down to a diffraction limited focus spot with a diameter of a few microns, and do so at all points of the scan-pattern within a surgical volume, or at all scanner positions of the surgical laser system. This high precision makes the modern laser surgical systems capable of maintaining the beam intensity marginally above the plasma threshold along the entire scan-pattern within the surgical volume in ideal or model targets.
Unfortunately, in spite of all the design and manufacturing effort spent on optimizing the laser sources and optics, the focus spot in the ophthalmic target region is often still larger than its diffraction limited value because the target tissue itself often gets distorted, making it different from the ideal or model targets used during the design of the laser optics. Distortions can be also caused by imperfections of the scanning delivery system and the focusing optics. The enlarging of the focus spot caused by any of these distortions can lead to failing surgical performance since it lowers the pulse energy density or beam intensity below the plasma threshold and thus prevents the scanning laser beam from forming the planned surgical cuts, leaving uncut lines or regions in the target region.
This problem of failing surgical performance can become particularly acute during surgical cuts where the targeted tissue is very thin such as a capsulotomy of the thin lens capsular bag during a cataract surgery. Since the targeted tissue is thin, the laser beam scans it only once or only a few times along a loop, as this scan-pattern should be already capable of cutting through the entire thickness of the capsular bag. However, if any one of the above distortions reduces the beam intensity below the plasma threshold along a section of the loop then that section can remain uncut. This uncut section of the capsular bag needs to be cut and separated manually, possibly leading to a tearing of the capsular bag and thus to a substantial lowering of the precision of the cataract surgery. Therefore, there is a need for surgical laser systems that can deliver the laser beam with a pulse energy density that is marginally higher than the plasma threshold in the entire surgical volume even if distortions are present along the beam path either in the target region or in the optical system itself, as such laser systems are capable of cutting the target region according to the scan-pattern in the entire surgical volume without leaving uncut regions or lines.